Charge transporting elements generally comprise a support and a charge transport layer across which charge moves under certain conditions. Charge transporting elements include electrophotographic charge generating elements.
In the use of such charge generating elements (also known as electrophotographic elements), incident light induces a charge separation across the various layers of the element. The electron and hole of an electron-hole pair produced within a charge generating layer separate and move in opposite directions to develop a charge between an electrically conductive layer and an opposite surface of the element. The charge forms a pattern of electrostatic potential (also referred to as an electrostatic latent image). The electrostatic latent image can be formed by a variety of means, for example, by imagewise radiation-induced discharge of a uniform potential previously formed on the surface. Typically, the electrostatic latent image is then developed into a toner image by contacting the latent image with an electrographic developer and the toner image is then fused to a receiver material. If desired, the latent image can be transferred to another surface before development or the toner image can be transferred before fusing.
The requirements of the process of generating and separating charge place severe limitations on the characteristics of the layers in which charge is generated and holes and/or electrons are transported. For example, many such layers are very soft and subject to abrasion. This places severe constraints upon the design of charge generating elements. Some configurations cannot provide a reasonable length of service unless an abrasion resistant overcoat layer (also known as "overcoat") is provided over the other layers of the element. This presents its own problems since charge must be able to pass through the overcoat.
The resistivity of an overcoat has major consequences in an electrophotographic system. If the overcoat has high resistivity, the time constant for voltage decay will be excessively long relative to the processing time for the electrophotographic element and the overcoat will retain a residual potential after photodischarge of the underlying photoreceptor. The magnitude of the residual potential depends upon the initial potential, the dielectric constants of the various layers, and the thickness of each layer.
A solution to this problem has been to reduce the thickness of the overcoat. Another solution is to provide an overcoat that is conductive. The overcoat must, however, not be too conductive. The electrophotographic element must be sufficiently electrically insulating in the dark that the element neither discharges excessively nor allows an excessive migration of charge along the surface of the element. An excessive discharge ("dark decay") would prevent the formation and development of the electrostatic latent image. Excessive migration causes a loss of resolution of the electrostatic image and the subsequent developed image. This loss of resolution is referred to as "lateral image spread" (evidenced as "image width"). The extent of image degradation will depend upon processing time for the electrophotographic element and the thickness and resistivities of the layers. It is thus desirable to provide an overcoat that is neither too insulating nor too conductive.
Silsesquioxanes are siloxane polymers, sometimes represented by the formula (RSiO.sub.1.5).sub.z, that are commonly prepared by the hydrolysis and condensation of trialkoxysilanes. Some of the polymers have been modified by the inclusion of polyethers or polydialkyloxysilanes. Generally, coatings of such materials are between 0.5 to 10 .mu.m thick and are applied from aqueous alcohol solvent systems. They have been commercially available from a number of sources for years (for example from Dow Corning, General Electric and Optical Technologies). A number of patents describe the use of such polymers to provide abrasion-resistant coatings for various purposes [see for example U.S. Pat. No. 4,027,073 (Clark), U.S. Pat. No. 4,159,206 (Armbuster et al), U.S. Pat. No. 4,277,287 (Frye), U.S. Pat. No. 4,324,712 (Vaughn, Jr.), U.S. Pat. No. 4,407,920 (Lee et al) and U.S. Pat. No. 4,923,775 (Schank)]. Typical uses of such polymers include scratch resistant coatings on acrylic lenses, photoreceptors and transparent glazing materials, and as overcoats for electrophotoconductive elements. For example, U.S. Pat. No. 4,159,206 (noted above) describes the use of neutral-charged, durable coating compositions that include colloidal silica and a mixture of dialkyldialkoxysilanes and alkyltrialkoxysilanes in a methanol/water solvent system. The mixture of silanes is believed to react to form silsesquioxanes.
Useful silsesquioxane-containing overcoats are described for example in U.S. Pat. No. 5,731,117 (Ferrar et al) and U.S. Pat. No. 5,693,442 (Weiss et al). In such layers, charge transport is provided by the presence of a charge canner that is complexed with the silsesquioxane.
Solid electrolytes (also known as solid ionic conductors) are solid materials in which electrical conductivity is provided by the motion of ions not electrons. A variety of solid electrolytes are inorganic crystals. Others are complexes of organic polymers and salts, such as complexes of poly(ethylene oxide) and alkali metal salts [see for example, Cowie et al, Annu. Rev. Phys. Chem., Vol. 40, (1989) pp. 85-113, Shriver et al, Chemical and Engineering News, Vol. 63, (1985) pp. 42-57, Tonge et al, Chapter 5 Polymers for Electronic Applications, ed. Lai, CRC Press, Boca Raton, Fla., 1989, pp. 157-210, at 162, and Cowie, Integration of Fundamental Polymer Science and Technology, Vol. 2, Elsevoir Publisher, New York, 21.5 (1988), pp. 54-62].
Electrical surface conductivities for polymeric and inorganic solid ion conductors are in the range of about 1.times.10.sup.-8 to 10 (ohms/square).sup.-1 [Surface conductivity is equal to conductivity divided by thickness and is expressed as (ohms/square).sup.-1 ]. Surface resistivity is equal to resistivity divided by thickness as expressed in ohms/square. For example, a resistivity of 1.times.10.sup.14 ohms-cm for a layer having a thickness of 5 .mu.m, equates to a surface resistivity of 2.times.10.sup.17.
Copending and commonly assigned U.S. Ser. No. 09/223,429 filed by Ferrar, Yoerger, Cowdery, Sinicropi, Parton and Weiss on Dec. 30, 1998, and entitled "Silsesquioxane Electrolytic Composition and Electrophotographic Charge Generating Element Containing Same", and U.S. Ser. No. 09/222,639 by Ferrar, Sorriero, Cowdery and Weiss on Dec. 30, 1998, now U.S. Pat. No. 6,066,425, and entitled "Electrophotographic Charge Generating Element Containing Primer Layer", describe improved electrophotographic charge generating elements that include overcoats containing silsesquioxane salt complexes. Such overcoats provide considerable advantages in resistance to damage from physical handling, corona discharge or other radiation sources, gases such as ozone, and chemicals such as nitric acid. If desired, the overcoats can be adhered to underlying photoconductor charge generating layers with improved primer layer formulations.
Heretofore, acid scavengers have been added to organic photoconductor layers only, as described for example in U.S. Pat. No. 5,368,967 (Schank et al). These layers are generally insulators that carry charge when either holes or electrons are injected into them. In the materials described in this patent, hydroxy-substituted triphenylmethane compounds are used as stabilizers to protect charge transport agents such as hydroxyarylamines.
In addition, EP-A-0 771,805, EP-A-0 771,809, U.S. Pat. No. 5,824,443 (Kushibiki et al), U.S. Pat. No. 5,688,961 (Kushibiki et al), and U.S. Pat. No. 5,712,360 (Kobayashi et al) describe the incorporation of triarylamines into the sol-gel network of siloxanes to provide charge transport.
There is a need, however, to further protect the electrophotographic charge generating elements containing silsesquioxane salt complexes from corona charging and to improve resistance to lateral image spread. We have found that this is possible with the present invention as described in more detail below.